Sloping masonry chimney (C) Daniel Friedman Angled or Sloped Chimney & Flues
Codes, Install, Inspect, Repair

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Angled chimneys & sloped flues: this article explains the hazards of angled or sloped masonry or metal chimneys and chimney flues.

Discussion includes code & safety issues with angled chimney flues - proper cutting of clay flue tiles for an angled flue; maximum angle for sloping chimney flues.

These articles on chimneys and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Angled and Offset Chimney Flues - Specifications & Concerns with Sloping Chimneys & Flues

Offset flue specifications (C) Carson Dunlop AssociatesThis article series describes chimney defects and hazards that can be observed from on-roof access, including damage to a masonry chimney top, antennas mounted on chimneys, and angled chimney flue hazards.

Our photo at page top shows a chimney that was angled severely beginning at the attic floor - look there for cracks that may have opened, forming a fire and gas hazard.

Offset or angled chimney flues are found in both masonry and metal chimney installations.

Especially where multiple flues are routed inside of a single large masonry chimney or inside of a single wood-framed chimney chase, it should not be a surprise to discover that one or more of the appliances venting into one or more of the flues has to send its gases up at an angle to reach the final vertical section of flue in the chimney.

Look for these clues that may indicate angled chimney flues that deserve further inspection:

Clay flue tile liners mitered when on angle (C) Carson Dunlop Associates

Just as we reported that a clay tile lined chimney would be difficult to clean due to projecting concrete between vertically-stacked clay chimney tiles, clay tiles need to be cut properly when constructing an angled chimney such as the one shown in Carson Dunlop Associates [at REVIEWERS] sketch.

The sketches shown at left shows the concern with proper miter joints in a sloping masonry chimney flue.

[Click to enlarge any image]

Only the bottom design, showing that the clay chimney tile was cut on an angle, is correct.

The top two designs risk both cleaning difficulties and also water, smoke, soot,or creosote leaks into the chimney structure.

Offset flue specifications (C) Carson Dunlop Associates The next Carson Dunlop sketch at left shows that a masonry flue is limited to 30 deg. offset, or in some jurisdictions, 45 degrees of offset while a metal flue may, in some areas, be permitted to slope to 60 degrees of offset.

In sum, concerns about angled or sloping chimneys and chimney flues include:

Support Rules for Sloped Chimneys & Elbows in Metal Chimneys & Flues

If elbows are used in the chimney the slope cannot be less than 30 degrees, and the sloping segment of the chimney above each elbow must be supported by straps or other means specified by the chimney manufacturer.

Question: can I slope the oil fired furnace flue pipe at 45 degrees? What about 30 degrees?

Metal chimney installation at 30 degree angle adapted from Selkirk 2100 insulated chimney installation instructions cited in this article (C) . Selkirk2018/02/11 John said:

I have an attached garage that I want to put a oil fired furnace in. I am curious if I’m allowed to put flue pipe at a 45 degree angle in attic space to get horizontal distance from house to the required 10 feet.

This question and reply were posted originally at CHIMNEY HEIGHT & CLEARANCE CODE

Illustration of a 30 degree chimney angle installation for an insulated metal chimney, adapted from Selkirk's installation instruction for the company's SuperVent 2100 cited below.

[Click to enlarge any image]

Reply: maximum number of 15° or 30° elbows and maximum non-vertical length for an insulated metal chimney indoors


We too found some sources warning that a 45° or 30° metal chimney might be out of compliance with UL 103 and may presented increased risk of chimney fire, particularly where soot or debris can accumulate in a hard-to-access sloping chimney section.

But installations to both 45° or 30° angles off from vertical (not from horizontal) are described by both standards and manufacturers.

If you going to proceed you should nevertheless review your installation plan with your local fire inspector.

To be clear, we are discussing a metal chimney installation, for an oil fired heating furnace, not a flue-vent connector or "stackpipe" - the term we use to describe the metal connector between the heating appliance and the actual chimney.

You would not run an uninsulated flue vent connector through an attic. A flue vent connector must slope at least 1/4" per foot of run. The heater manufacturer may have other specifications and limitations on flue vent connector length. We discuss flue vent connectors separately at FLUE VENT CONNECTORS, HEATING EQUIPMENT.

Reader follow-up:

How about 30 degrees I read on another forum that was max

Reply: Example Manufacturers' Metal Chimney Slope, Length, Elbows, Angle Specification

A 30° metal chimney is when that angle is offset from vertical, closer to vertical than 45. Both can be acceptable provided you follow both the manufacturer's installation instructions and your local building code requirements. You'll see from an example we give below that depending on where you live there may be different constraints on permissible chimney slope, elbows, and non-vertical length.

You will also see that 15° offsets are described by various manufacturers.

Here is a typical insulated metal chimney installation guide from a manufacturer.

One pair of (two) 15° or 30° elbows may be used in an interior installation to provide an offset in order to avoid cutting of joists and to clear other obstructions.

The maximum permissible angle with solid fuel installation is 30 degrees.

The vertical run of chimney above an offset must be supported with an elbow support.

Each elbow support will support 15 ft. (4.5 m) of chimney and the maximum length of chimney allowed between the elbows is 6' (2m).

Refer to the Offset Chart 1 on page 13 in these instructions for more details. Elbow kits contains 2 Elbows, 4 Locking Bands and an Elbow Support. 

- source: SUPER VENT 2100 & SUPER PRO 2100 INSTALLATION INSTRUCTIONS & MAINTENANCE GUIDE [PDF] retrieved 2018/02/12, original source: and cited in more detail below.

Example State Code on Chimney Offsets & Pitch

Here is a typical chimney or flue-vent connector slope (or pitch) guideline from a U.S. state code. There you will see that although the manufacturer may permit slopes of less than 45° a particular state guideline might not.

Pitch and length. Chimney or vent connectors shall have no more than two 45° offsets with the vertical. The horizontal length shall not exceed 75% of the total vertical height of the total venting system measured from the appliance outlet.

Chimney or vent connectors shall be pitched at least 1/4-inch per foot from the appliance outlet collar vent to the chimney inlet.

- source: U.S. state of Wisconsin, SPS 323.15 - Chimney connectors, smoke pipes and stovepipes. found in Wisconsin SPS-323, HEATING, VENTILATING AND AIR CONDITIONING [PDF] cited in detail below.

Background: Excerpt from UL-103 description

This category covers chimney and vent connectors and accessories which are engineered systems intended for connecting appliances to a vertical chimney or vent, or to reduce the clearances from connectors to combustible material in accordance with ANSI/NFPA 211, "Chimneys, Fireplaces, Vents and Solid Fuel-Burning Appliances," and ANSI/NFPA 54, "National Fuel Gas Code," or applicable local building code requirements.

The type(s) of fuel-fired heating appliances (e.g., gas, liquid, solid fuel) with which these connectors and accessories have been investigated are indicated in the individual certifications. This information, together with other restrictions of use, such as mounting position, are also marked on the product and/or detailed in the manufacturer's installation instructions furnished with the product. - retrieved 2018/02/12, original source: UL-DDCY.GuideInfo Chimney and Vent Connectors and Accessories - source:

U.S. Chimney Codes & Standards Addressing Sloped or Angled Flues & Chimneys


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